As the integration densities of semiconductor devices continue to increase, the widths of gate electrodes are generally reduced. As the width of a conventional doped polysilicon gate electrode approaches 0.25 microns (.mu.m), however, its performance may deteriorate. In particular, as the resistance of the polysilicon gate increases with the reduction of the width, the transmission of a gate signal may thus be delayed. Furthermore, when a sufficiently narrow polysilicon gate is used in a p-MOS transistor, short channel effects may increase because of the buried channel formed to lower the threshold voltage.
In order to reduce the effects of the above mentioned problems, the formation of gate electrodes from a conductive material having a relatively low resistivity and a work function corresponding to the midgap of silicon have been investigated. In particular, it has ben noted that a polycide structure formed from a silicide (a compound formed of metal and silicon which are heat-treated) and polysilicon can be used as a gate electrode material in place of a doped polysilicon structure.
The polycide structure can include tungsten silicide (WSix) or titanium silicide (TiSix) as the silicide. Alternately, a silicide including a metal having a relatively high melting point, such as a cobalt silicide (CoSix) for example, can be used. Of the above mentioned silicides, titanium silicide (hereinafter referred to as TiSix) excels in thermal stability and has a relatively low resistivity which is about one quarter that of tungsten silicide (WSix). TiSix is thus considered a very suitable material for a gate electrode, and in particular for a gate electrode for a device such as a dynamic random access memory (DRAM) with a memory capacity of over 1 gigabit.
FIGS. 1A to 1C are views illustrating steps of a method for forming a gate having a Ti-polycide structure, according to the prior art. As shown in FIG. 1A, a gate oxide layer 4 is formed on the surface of a semiconductor substrate 2, and a conductive polysilicon layer 6 is formed on the gate oxide layer. A titanium layer is then deposited on the polysilicon layer 6, and this structure is heat-treated so that the polysilicon and titanium react with each other to form a titanium silicide (TiSix) layer 8. Alternately, the titanium silicide (TiSix) layer can be directly deposited on the polysilicon layer 6 using a sputtering or other technique.
An insulating layer 10 can be formed by depositing a layer of silicon oxide or silicon nitride on the titanium silicide (TiSix) layer 8, as shown in FIG. 1A. A patterned photoresist layer 12 is then formed on the insulating layer to define the gate pattern, and this photoresist layer can be patterned using conventional photolithographic techniques, as shown in FIG. 1B. The insulating layer 10, the titanium silicide layer 8, and the polysilicon layer 6 are then sequentially etched using the patterned photoresist layer 12 as an etching mask, as shown in FIG. 1C. A gate structure having a polycide structure including the titanium silicide layer 8 and the polysilicon layer 6 can thus be provided.
The titanium silicide and polysilicon layers which make up the gate pattern can be etched using a dry etching technique. In particular, a fluorine-series gas (e.g., sulfur hexafluoride SF.sub.6 or carbon tetrafluoride CF.sub.4), a chlorine-series gas (e.g., hydrogen chloride HCl, chlorine gas Cl.sub.2 or boron trichloride BCl.sub.3), and/or hydrogen bromide HBr can be used as the etching gas. The TiSix layer 8 and the polysilicon layer 6, however, may be susceptible to sidewall erosion during the etch. This sidewall erosion of the gate structure may cause a bridge to form between conductive layers due to a stringer phenomenon wherein a conductive material remains at the eroded portion after the next processing step (e.g., a pad electrode formation step). The reliability of the semiconductor device may thus be decreased and product yield reduced.
The gate oxide layer may also be damaged because fluorine-series gasses and boron trichloride (BCl.sub.3) have low etching selectivities with respect to the oxide. In addition, when hydrogen bromine (HBr) is used as the etching gas, it may react with the TiSix layer 8 to produce a significant amount of polymer (i.e., a non-volatile residue) which may remain between the gate electrodes being patterned making it difficult to adjust the critical dimension thereof. Furthermore, when only chlorine gas (Cl.sub.2) is used as the etching gas with a hard mask instead of photoresist, severe sidewall erosion may occur.